Generally speaking, wet friction members having high porosities are preferred for improving a dynamic friction performance of a wet clutch. Those achieving high static friction performances are also preferred for improving a torque capacity of a clutch system.
Materials for wet friction members containing carbon fibers, which are called carbon fiber composite materials, are known in the art (e.g., see JP 11-5850 A). Such carbon fiber composite materials may be manufactured with a dry process or an impregnation process. In the dry process, a mixture of carbon fibers and resin powder is prepared at the outset. Subsequently, the mixture is molded and then subjected to heat. At this time, the resin powder melts and solidifies to bind the carbon fibers together, while becoming carbonized into a matrix, whereby a carbon fiber composite material is obtained. In the impregnation process, on the other hand, a carbon fiber woven or nonwoven fabric is impregnated with a resin solution. Subsequently, the carbon fiber woven fabric or the like is dried and then subjected to heat. Accordingly, the resin contained in the carbon fiber woven fabric or the like becomes carbonized into a matrix, whereby a carbon fiber composite material is obtained.
In order to improve both of the dynamic friction performance and the static friction performance in a wet friction member comprising a carbon fiber composite material, the proportion of the carbon fibers therein should be increased and the porosity thereof should be increased. In other words, the wet friction member should have a porous structure with a larger volume of carbon fibers bound together by a smaller quantity of matrix.
However, the manufacture of a wet friction member through the dry process or impregnation process as previously employed would require an excessive amount of resin to be used for the carbon fibers, because the carbon fibers are rigid and less flexible, hardly tangled together, and of low bending stiffness. As a result, a volume fraction of carbon fibers (exclusive of pores) in the wet friction member would be low.
Moreover, another disadvantage in a wet friction member obtained through the conventional dry or impregnation process is that distribution of carbon fibers, matrix and pores would become uneven. As a result, the conventional dry or impregnation process could not produce a wet friction member having a large volume of carbon fibers and a high porosity. Accordingly, the conventional dry or impregnation process could not remove difficulties in manufacturing a wet friction member achieving excellence in both of the static friction performance and the dynamic friction performance.
With this in view, it would be desirable to provide a wet friction member achieving excellence in both of the static friction performance and the dynamic friction performance, a carbon fiber composite material from which such a wet friction member is made, and a process for manufacturing the same.